The radiolabeled microsphere technique has been utilized to demonstrate the regional distribution of coronary blood flow within an isolated supported dog heart when total coronary blood flow has been maintained constant. We have shown that the sequential administration of Ce141 and Sr85 labeled plastic microspheres of approximately 15 microns diameter in the same heart does not alter significantly the myocardial distribution of the second microsphere. The effect of norepinephrine infusion upon epicardial/endocardial blood flow ratios has been studied with the radiolabeled microsphere technique. Our results indicate that norepinephrine redistributes or shunts blood flow from endocardial to epicardial areas of the myocardium. We propose to determine the mechanism by which this alteration in regional blood flow occurs. The effect of heart rate, left ventricular systolic pressure and coronary artery perfusion pressure upon regional blood flow in the myocardium will be studied independently. We will also evaluate the effect of beta adrenergic blocking agents upon the norepinephrine induced reduction in endocardial blood flow.